This invention relates to an improved process for crystallizing amorphous polyester granules of any shape or size so that the granules do not stick together in a subsequent process such as solid state polymerization. The granules crystallization process of this invention is also a useful pre-treatment to facilitate a melt extrusion process and provide more uniform extruded products.
In searching the Prior Art for this invention it is useful to note that:
(1) Polyester Granules are also described in Pellets, Chips or Particulate Polyester with at least one dimension of the solid particle greater than one millimeter.
(2) Some crystallization of polyester granules may take place in processes such as Drying, Heat Setting, Annealing or a Heat Process used to achieve Dimensional Stability.
(3) The Polyester Granules of this invention are Polyethylene Terephthalate or simply PET.
One method of preventing granules sticking together discussed in the prior art involves coating the granule surfaces with some xe2x80x9cnon-stickxe2x80x9d material.
U.S. Pat. No. 3,544,523 (Mobil Oil) teaches that polyester granules may be prevented from sticking together by coating the granules with a small amount of powdered anti-caking agent such as talc.
U.S. Pat. No. 5,919,872 (Shell) adds an alkylene carbonate to the polyester to reduce the tendency of the polyester pellets to stick to-gether in a solid state polymerization process in which the pellets are not agitated.
My U.S. Pat. No. 4,731,281 (Fleischer and Palmer) describes a process for encapsulating polyester monofilaments with a coat of an anti-sticking polymer such as an addition of 0.5% of a fluorocarbon polymer to the surface of the monofilaments. This coating process may be used to coat the polyester strands (spaghetti) prior to cutting into pellets.
All of the above three prior art patents use an additive to the PET which may not be suitable in subsequent processes. For example the additive may tend to block the orifices in a die or spinneret. Additionally the additive may be detrimental to the performance of the extruded product (e.g. soft-drink polyester bottles.) It is therefore of interest to use granule crystallization processes that use only heat, and no additive, to achieve non-stick properties of the PET granules.
A simple Drying Process of PET granules at 150xc2x0 C. for one hour will produce some crystallinity in the granules. However the granules may still stick together and aggregate in a lump if the granules are subjected to a post-crystallization process above 150xc2x0 C. without provision of stirring or agitation of the PET granules.
U.S. Pat. No. 4,161,578 (Bepex) describes a crystallization process for PET granules in which temperatures in a range 180xc2x0 C. to 220xc2x0 C. are used with residence times in the crystallizer of less than 0.5 hour. This higher temperature process may prevent PET granules from sticking together in the heated hopper or the compacting zone of a screw extruder. However, contrary to the claims of ""578 patent, a crystallization temperature of 220xc2x0 C. may not be high enough to prevent PET granules sticking together in some Solid State Polymerization Processes where the granules reach 230xc2x0 C. without agitation. One feature of patent ""578 is that crystallization of PET granules of different sizes were investigated.
U.S. Pat. No. 4,064,112 (Zimmer/Eastman) addresses the problem of crystallizing PET granules so that they do not stick together in a solid state polymerization process operating at 230xc2x0 C. without granule agitation. Patent ""112 describes a process for crystallizing PET granules at 230xc2x0 C. for 0.5 to 3 hours. Patent ""112 teaches that the crystallization temperature should be equal to, or higher, than the solid-stating temperature. This procedure of patent ""112 is consistent with the Heat Setting Technology of Polyester Yarns and Fabrics which states that to obtain good Dimensional Stability of a textile structure it is necessary to heat set or anneal the structure at a temperature equal to, or higher, than the temperature it is exposed to in a subsequent process or in normal end use. However due to the long residence time, typically an hour or more, of process ""112 it is necessary to use hot nitrogen in the crystallizer (instead of hot air) to prevent degradation of the PET granules. Additionally patent ""112 crystallizes the PET granules throughout their volume which requires higher temperatures in a subsequent extrusion process.
U.S. Pat. No. 5,714,571 describes a process for crystallization of PET resins in a fluid bed crystallizer where the nitrogen fluidizing gas enters the crystallizer at a temperature not lower than 195xc2x0 C. The residence time of the granules in the fluid bed crystallizer is higher than 5 minutes. Since Eng. owns ""571.
U.S. Pat. No. 5,744,074 (Du Pont) produces PET polymer from an amorphous polyester melt. Crystallization temperatures as high as 230xc2x0 C. are quoted. Whilst very short residence times are mentioned in ""074, one objective was to achieve a more uniform crystallization throughout each PET pellet.
U.S. Pat. No. 5,119,570 (Buhler AG) does suggest a stage of polyester crystallization in which the granule is xe2x80x9ccrystallized in part and has a fraction of amorphous material.xe2x80x9d ""570 uses 2 fluid beds.
The Prior Art of Crystallizing PET Granules may be summarized by considering the following process conditions or process features:
(1) Temperatures of 230xc2x0 C. or higher
(2) Short Residence Time in Crystallizer (less than 10 minutes for small PET granules).
(3) Use of hot air instead of the more expensive Nitrogen
(4) Production of a bi-component PET granule with a crystalline skin and essentially amorphous interior.
(5) Use of PET granules which facilitate the melt extrusion process.
In addition to preventing the PET granules from sticking together, Prior Art has recognized the importance of using a PET granule Crystallization Process which facilitates post-crystallization processes and/or improves the quality of the products extruded from the crystallized PET granules.
For example:
(1) U.S. Pat. No. 4,064,112 teaches that the use of higher solid state temperatures, without the PET granules sticking together, increases solid-state polymerization productivity.
(2) U.S. Pat. No. 5,843,573 (Nippon) teaches the production of a polyester resin composition which xe2x80x9celiminates a precise temperature control during a molding pressure, and saves labor force resulting in desirable moldability and productivity.
(3) My U.S. Pat. No. 5,753,168 (Palmer) recognized the importance of having a PET granule which could be used in a melt extrusion process capable of producing polyester monofils of very uniform diameters.
As shown in the Table above the only crystallization process capable of preventing the PET granules sticking together and providing improved post-crystallization processes and products is the Crystallization Process used in this Invention.